Shaft-sealing system for nuclear turbines

ABSTRACT

A nonradioactive source of steam for sealing the shaft packings of turbines operating on steam subject to nuclear contamination is generated by a special steam seal evaporator. The nonradioactive steam is supplied to the high-pressure shaft packing chamber as a buffer between contaminated steam and the atmosphere, and prevents mixing of contaminated steam with air extracted from the packings. The evaporator is heated by steam extracted from the turbine for maximum cycle efficiency. Clean sealing steam is generated at a pressure above atmospheric and eliminates conventional steam seal regulator devices. The system eliminates radioactive contamination of the air drawn from the turbine packings, reducing the size of the station off-gas system.

United States Patent Inventor Frederick G. Baily Ballston Spa, N.Y.

Appl. No. 749,077

Filed July 31, 1968 Patented Sept. 14, 1971 Assignee General ElectricCompany SHAFT-SEALING SYSTEM FOR NUCLEAR TURBINES 5 Claims, 3 DrawingFigs. 7

US. Cl

TYPICAL VALVE STEM 4o t! Primary Examiner-Carroll B. Dority, Jr.Attorneys-William C. Crutcher, Frank L. Neuhauser, Oscar B. Waddell andMelvin M. Goldenberg ABSTRACT: A nonradioactive source of steam forsealing the shaft packings of turbines operating on steam subject tonuclear contamination is generated by a special steam seal evaporator.The nonradioactive steam is supplied to the highpressure shaft packingchamber as a buffer between contaminated steam and the atmosphere, andprevents mixing of contaminated steam with air extracted from thepackings. The evaporator is heated by steam extracted from the turbinefor maximum cycle efficiency. Clean sealing steam is generated at apressure above atmospheric and eliminates conventional steam sealregulator devices. The system eliminates radioac tive contamination ofthe air drawn from the turbine packings, reducing the size of thestation off-gas system.

PATENTED SEP1 412m SHEET 1 BF 2 INVENTOR'.

FREDERICK 5. BAILY, BY @7 W ms ATTORNEY.

PATENTED SEP 1 4 I971 SHEET 2 OF 2 023.041 m J UE P INVENTOR: FREDERICKe. BAILY, BY 6 @w/w HIS ATTORNEY.

SHAFT-SEALING SYSTEM FOR NUCLEAR TURBINES BACKGROUND OF THE INVENTION Asteam-sealing system is provided in steam turbine powerplants to preventthe leakage of steam along the turbine shaft into the atmosphere in thecase of high-pressure packings, and to prevent the leakage of air alongthe shaft from the atmosphere into the turbine in the case of thelow-pressure packings. In conventional systems, the high-pressure andlow pressure packings receive steam from or contribute steam to a steamseal header which is held at a constant pressure by a steam sealregulator." Since the header and highand lowpressure packings are allinterconnected, and since the makeup to the steam seal header issupplied by working steam, any contamination in the main steam flowingthrough the turbine is also present in the steam-sealing system.

In the case of nuclear turbines operating on steam from nuclear reactorsof the open cycle type known as boiling water" reactors, where the steamgoes directly from the reactor to the turbine, the steam may contain atrace of radioactive, noncondensible gases from the reactor. Theseradioactive gases would be present in a conventional steam seal systemas well. When air from the atmosphere is drawn in along the shaftpackings, as is the usual practice, the leakoff steam from the packingis mixed with a much greater volume of air, possibly contaminating itwith a trace of radioactive noncondensible gas. This large volume ofair, although of very low radioactivity, must be treated as radioactivegas. Consequently, additional equipment may be required to treat largevolumes of air which have been slightly contaminated by thesteam-sealing system.

Accordingly, one object of the present invention is to provide animproved steam-sealing system for nuclear turbines operating on steamfrom the boiling water reactor.

A further object of the invention is to provide an improved sealingsystem utilizing clean steam as a buffer between the atmosphere andcontaminated steam in the turbine.

Still another object of the invention is to provide an improvedapparatus for supplying sealing steam to shaft packings of a turbine.

DRAWING The subject matter which is regarded as the invention isparticularly pointed out and distinctly claimed in the concludingportion of the specification. The invention, however, both as toorganization and method of practice, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription, taken in connection with the accompanying drawing, inwhich:

FIG. 1 is a simplified schematic illustrating the essential elements ofthe steam sealing system for a nuclear turbine powerplant,

FIG. 2 is a simplified diagram of a prior art-type of steam seal system,and

FIG. 3 is a comparable simplified diagram of the present steam seal.

SUMMARY OF THE INVENTION Briefly stated, the invention is practiced byproviding a steam seal evaporator to generate clean sealing steam, usingheat obtained from the main powerplant steam which may be contaminated.The clean steam is interposed in a shaft packing arrangement as a bufferand at a pressure slightly above atmospheric so that it flows into thecontaminated steam system or flows as clean leakoff steam to be mixedwith air and requiring no special treatment.

DESCRIPTION Referring to FIG. I of the drawing, a typical nuclear steamturbine powerplant comprises a high-pressure turbine I havinghigh-pressure shaft packings 2, 3 and one or more lowpressure turbines 4having substantially identical low-pressure shaft packings 5. Packings2, 3, 5 have inner ends communicating with the turbine sections andouter ends communicating with the atmosphere as indicated by the flowarrows.

Steam for turbines l, 4 is generated by a nuclear reactor 6 of theboiling water type and its flow to the turbines is controlled by one ormore control valves 7 and intermediate valves 8. Although the pressures,flows and number of turbine sections will vary with the particularpowerplant, inlet pressure to turbine I may be on the order of 1,000p.s.i. and exhaust pressure on the order of 200 p.s.i. After removal ofmoisture in separator 9, and possible reheating (not shown here), steamenters turbines 4 at a pressure on the order of 200 p.s.i. and exhauststo condensers I0 at a pressure on the order of 2 inches Hg. Abs., whereit is condensed to feedwater.

The feedwater, indicated by dot-dash lines, may include passage throughvarious heat recovery elements such as special auxiliary condensers 11,and includes passage through a series of feedwater heaters 12-16 ofincreasing shell pressure and supplied with steam removed from suitableextraction locations. Although in actuality there are numerousinterstage extractions for various purposes, the ones shown which arematerial to the present invention are an extraction conduit 17 from theexhaust end of the high-pressure turbine and extraction conduits 18 and19 from intermediate and low-pressure stages respectively of thelow-pressure turbine. These are referred to hereinafter ashigh-pressure, intermediate pressure and low-pressure extractionconduits for purposes of differentiation, although it is understood thatthese are merely relative terms rather than designating any particularpressure. The only absolute pressure connotation that these terms haveis that low-pressure extraction 19 is at a subatmospheric pressureroughly on the order of 5 p.s.i.a., this also being the shell pressureof the lowest feedwater heater 12. Feedwater is returned to reactor 6 tocomplete the closed cycle.

In accordance with the invention, a steam-sealing system with steampiping, shown in solid lines in FIG. 1, comprises a submerged tubeevaporator 20, a drain tank 21, a sealing steam supply header 22, aleakoff steam exhaust header 23, a subatmospheric pressure header 24, asteam seal exhaust condenser 25 and a fan, or blower, 26 exhausting tothe atmosphere.

The evaporator 20 is supplied with a clean or uncontaminated watersupply via line 27, and serves to evaporate the water to supply cleansealing steam to header 22, In normal operation, heating coils 28 forthe evaporator receive steam via valve 29 from intermediate pressureextraction conduit 18. The coils 28 drain via line 30 to tank 21. Fromthere the condensed fluid drains via valve 31 to the low pressurefeedwater heater 12.

Under alternate modes of operation, the heating coils 28 may also besupplied with main throttle steam via valve 32. Alternate drainage isobtained via a high level" valve 33 provided on drain tank 21 andconnected to the main condenser.

In order to control the heat energy supplied to submerged coils 38 andto obtain the heat energy under the most economical cycle conditions, apressure-sensitive control 34 causes heating steam to be normallyadmitted via valve 29 from extraction conduit 18, but when theextraction pressure is insufficient, such as during startup, controller34 causes the deficiency to be made up from main steam via valve 32.

An automatic float valve 35 controls the level of clean water suppliedto the steam seal evaporator 20. The generated steam is supplied via anautomatic pressure control valve 36 to header 22, responsive to pressurein the header. Valve 36 is controlled to keep the supply header pressureat a constant value slightly above atmospheric pressure, i.e., on theorder of 19 p.s.i.a.

The various steam seal headers 5 so connected to high-pressure packings2, 3 and low-pressure packings 5 so that the steam flows as indicated bythe arrows and to be more clearly explained in connection with FIGS. 2and 3. However, it is noted that the system is not limited to rotatingshaft packings,

but is applicable also to valve stem packings such as would be foundalong the stems of valves 7 and 8 to prevent steam leakage at theselocations.

A typical stem segment is indicated at 40 in FIG. 1. The upper end is atatmospheric pressure, and the lower end is exposed to the internal steampressure. It is believed that no further explanation of the sealingsystem as applied to a valve stem is necessary, since its operation asdescribed more particularly in connection with FIGS. 2 and 3 isessentially the same as the shaft-packing system, and for the samepurpose.

Referring to FIGS. 2 and 3 of the drawing, a prior art sealing systemfor typical high-pressure and low-pressure packings is illustrated inFIG. 2. The improved sealing system is comparably arranged in FIG. 3.The drawings of FIGS. 2 and 3 are stylized in order to more easilyillustrate the operation, but they will be understood by those skilledin the art. The portions along the top, such as 41, represent theturbine shafts (or valve stems), while the small rectangles such as 42represent packing rings, either smooth or having a series of annularteeth or a labyrinth. Each packing ring 42 forms a close clearance withthe turbine shaft or valve stem portions. The spaces 43 between thepacking rings 42 represent suitable leakoff or supply chambers.

Air flow is indicated by the open arrowheads, while steam flow isindicated by the solid arrowheads. The high-pressure packings on theleft necessitate a net flow of leakage steam out of the unit along theshaft, while the low pressure packings on the right necessitate a netflow of leakage steam along the shaft into the unit. In the diagrams,the dot-dash lines represent main steam from the powerplant which maycontain contaminates such as radioactive noncondensible gas from theboiling water reactor. In FIG. 3, the separate steam seal flow of buffersteam from the steam seal evaporator 20 is indicated in solid lines.

Referring first to a conventional steam seal system of FIG. 2 for asteam powerplant of this type, a steam seal header 44 was connected to asteam seal regulator 45 and the steam seal header was also connected tothe high-pressure packing 46 and the low-pressure packing 47. Thehigh-pressure packing contributed sealing steam to header 44 at highloads. Lowpressure packings 47 required steam from the header at allloads. At light loads, the sealing steam deficit was made up withthrottle steam admitted to leader 44 from regulation 45. At high loadsexcess steam was dumped to a suitable disposal point in the system suchas a low-pressure feedwater heater, by regulator 45. Air from thepowerplant station was drawn in along the shaft from the outer end ofthe packing by a moderately low subatmospheric pressure maintained by anexhauster consisting of elements such as 25, 26 (see FIG. 1). Thestation air was mixed in the steam packing chambers 48 with leakagesteam passing along the shaft in the opposite direction. A relativelylarge volume of air was thus mixed with the main steam leakoff from thepacking. If the main steam were contaminated, the mixture passing to theexhauster had to be treated as contaminated also, even though it wasgreatly diluted by air.

Referring to FIG. 3 of the drawing, the invention is seen to interposeclean sealing steam between the contaminated steam in the turbine andthe station in the case of the highpressure packing, and to supply cleansealing steam to the lowpressure packing. Specifically, thehigh-pressure packing comprises a high-pressure return chamber 49, alow-pressure return chamber 50, a clean steam supply chamber 51, and aclean leakoff chamber 52.

Similarly, the low-pressure packing comprises a clean steam supplychamber 51 and a clean leakoff chamber 52 performing the same functionsas in the high-pressure packing.

In both prior art (FIG. 2) and the present invention (FIG. 3), thehigh-pressure return chamber 49 is connected to highpressure extractionline 17, in the present case, at about 200 p.s.i. In the presentinvention, the low-pressure return chamber 50 is connected via header 24to low-pressure extraction line 19 leading to the lowest feedwaterheater 12 at subatmospheric pressure (about 5 p.s.i.a. in the presentcase). Thus, radioactive steam at main throttle pressure of 1,00 p.s.i.entering the inner end of the packing from the left will flow along theshaft and leave the packing via return chambers 49, 50.

The clean steam supply chambers 51 of both high pressure and lowpressure packings are supplied with steam from the steam seal evaporatorat a pressure slightly above atmospheric. The clean leakoff chambers 52are maintained at a pressure slightly below atmospheric by blower 26.Thus clean steam from chambers 51 will leak in both directions along theshaft to the subatmospheric chambers on either side. In the case of thehigh-pressure packing, a portion of the sealing steam will flow towardthe turbine as far as chamber 50. The other portion will be mixed withstation air and exhausted through the condenser 25 and blower 26.

In the case of the low-pressure packing, the steam flowing toward theturbine enters the main steam flow, while the leakoff steam from thechamber 32 is exhausted to the steam seal condenser.

OPERATION The operation of the system shown in FIG. 1 of the drawing isas follows: The sealing steam generated by steam seal evaporator 20 andsupplied to header 22 is completely clean and independent of the mainsteam flow in the turbine. It is generated, however, by working steampassing through the heating coils 28 suitably disposed to receive steamfrom an intermediate pressure extraction line. The condensed extractionsteam flows to the lowest feedwater heater held at subatmosphericpressure by a low-pressure steam extraction line.

Thus the suitable selection of two extraction pressures between which toplace the steam seal evaporator 20 can be used to determine a relativelyhigh thermodynamic efficiency of the sealing system with respect to theoverall cycle. In addition, the supply of steam to the heating coils 28can be augmented by throttle steam under unusual circumstances by propercontrols.

The introduction of a clean sealing steam as a buffer between thestation air and the main steam flow provides the ability to treatleakoff steam from the shaft packings and the valve stem packings in themanner previously accorded to leakoff steam from fossil fired steamturbine powerplants without other special precautions.

While there is shown what is considered at present to be the preferredembodiment of the invention, it is of course understood that variousother modifications may be made therein, and it is intended to cover inthe appended claims all such modifications as fall within the truespirit and scope of the invention.

What I claim is:

1. In a steam turbine subject to receiving contaminated steam in aclosed steam and feedwater cycle and having at least one high-pressurepacking for sealing a turbine section operating above atmosphericpressure and at least one lowpressure packing for sealing a turbinesection operating below atmospheric pressure, said packings defining aplurality of axially spaced chambers between groups of packing teethforming close clearances with the a, each of said packings having anoutermost end communicating with the atmosphere through one of saidclearances, a noncontaminating shaft seal system for one of saidpackings comprising:

first means including an exhauster system maintaining a first cleanleakoff packing chamber at subatmospheric pressure adjacent saidoutermost end of the packing, and communicating with the atmosphere,

second means maintaining a second steam return chamber at asubatmospheric pressure, said steam return chamber also communicatingwith the turbine cycle steam, and third means generating clean sealingsteam at a pressure above atmospheric and supplying it to a thirdpacking chamber between the first and second chambers, said clean steamgenerating means comprising an evaporator supplied with uncontaminatedwater and heated by steam extracted from said closed steam and feedwatercycle, whereby a portion of the generated clean steam flows toward theturbine and a portion is drawn along with air leaking inward from theoutermost end of the packing through said exhauster system.

2. The combination according to claim 1, wherein said evaporatorcomprises a submerged coil connected at one end to a steam extractionline and at the other end to a tank draining to a lower pressure pointin the steam and feedwater cycle.

3. The combination according to claim 1, wherein said steam returnchamber is disposed in said low-pressure packing and communicatesdirectly with said low-pressure turbine section.

' 4. in a steam turbine subject to receiving radioactive steam at leastone low-pressure packing having at least the follow ing packing chambersin succession from the outer end of the packing, a. a subatmosphericclean steam leakoff chamber, b. a clean steam supply chamber maintainedabove atmospheric pressure,

a submerged tube steal seal evaporator having a clean water supplysource,

first conduit means arranged to supply said submerged tube evaporatorwith extraction steam from the turbine,

a drainage tank for collecting condensed water from said submerged tubeevaporator,

means controlling the flow of steam from said evaporator to said cleansteam supply chambers and arranged to hold the pressure thereinsubstantially constant,

' means'connecting said return chamber of said high-pressure.

packing to a subatmospheric point in the cycle, and

a leakoff steam exhaust system connected to withdraw a mixture of airand steam from said clean leakoff chambers and vented to the atmosphere.

5. The combination according to claim 1, wherein seal system for saidpacking also includes a high-pressure steam return chamber disposedadjacent to and on the turbine section side of said second steam returnchamber, said high-pressure chamber being connected to a point in saidclosed steam and feedwater cycle to maintain the pressure thereinbetween that of said second steam return chamber and that of thesection.

1. In a steam turbine subject to receiving contaminated steam in aclosed steam and feedwater cycle and having at least one highpressurepacking for sealing a turbine section operating above atmosphericpressure and at least one low-pressure packing for sealing a turbinesection operating below atmospheric pressure, said packings defining aplurality of axially spaced chambers between groups of packing teethforming close clearances with the a, each of said packings having anoutermost end communicating with the atmosphere through one of saidclearances, a noncontaminating shaft seal system for one of saidpackings comprising: first means including an exhauster systemmaintaining a first clean leakoff packing chamber at subatmosphericpressure adjacent said outermost end of the packing, and communicatingwith the atmosphere, second means maintaining a second steam returnchamber at a subatmospheric pressure, said steam return chamber alsocommunicating with the turbine cycle steam, and third means generatingclean sealing steam at a pressure above atmospheric and supplying it toa third packing chamber between the first and second chambers, saidclean steam generating means comprising an evaporator supplied withuncontaminated water and heated by steam extracted from said closedsteam and feedwater cycle, whereby a portion of the generated cleansteam flows toward the turbine and a portion is drawn along with airleaking inward from the outermost end of the packing through saidexhauster system.
 2. The combination according to claim 1, wherein saidevaporator comprises a submerged coil connected at one end to a steamextraction line and at the other end to a tank draining to a lowerpressure point in the steam and feedwater cycle.
 3. The combinationaccording to claim 1, wherein said steam return chamber is disposed insaid low-pressure packing and communicates directly with saidlow-pressure turbine section.
 4. In a steam turbine subject to receivingradioactive steam from a boiling water reactor and in a closed cycle andhaving shaft packing defining a plurality of axially spaced packingchambers between groups of packing teeth forming close clearances withthe shaft, a noncontaminating shaft sealing system comprising: at leastone high pressure packing having at least the following packing chambersin succession from the outer end of the packing, a. a subatmosphericclean steam leakoff chamber, b. a clean steam supply chamber maintainedabove atmospheric pressure, c. a subatmospheric pressure contaminatedsteam return chamber, at least one low-pressure packing having at leastthe following packing chambers in succession from the outer end of thepacking, a. a subatmospheric clean steam leakoff chamber, b. a cleansteam supply chamber maintained above atmospheric pressure, a submergedtube steal seal evaporator having a clean water supply source, firstconduit means arranged to supply said submerged tube evaporator withextraction steam from the turbine, a drainage tank for collectingcondensed water from said submerged tube evaporator, means controllingthe flow of steam from said evaporator to said clean steam supplychambers and arranged to hold the pressure therein substantiallyconstant, means connecting said return chamber of said high-pressurepacking to a subatmospheric point in the cycle, and a leakoff steamexhaust system connected to withdraw a mixture of air and steam fromsaid clean leakoff chambers and vented to the atmosphere.
 5. Thecombination according to claim 1, wherein seal system for said packingalso includes a high-pressure steam return chamber disposed adjacent toand on the turbine section side of said second steam return chamber,said high-pressure chamber being connected to a point in said closedsteam and feedwater cycle to maintain the pressure therein between thatof said second steam return chamber and that of the sectIon.